The present invention relates to the production of polyolefins in a multiple stage fluidized bed gas phase polymerization system.
The use of gas phase fluidized bed polymerization systems in the production of polyolefins is well known in the prior art, as shown, for example, by U.S. Pat. Nos. 3,023,203, 4,003,712, and 4,588,790. In such systems, gaseous alpha-olefins typically having two to twelve carbon atoms, and more particularly ethylene and/or propylene are introduced into a reactor having a plenum, a polymerization zone and a disengaging zone. A fluidizing gas mixture comprising the alpha-olefin is supplied to the plenum of the reactor, from which the gas travels upwardly through a gas distribution plate into the reactor polymerization zone. Prior art gas distribution plates are flat and have a plurality of ports therein to distribute the flow of polymerization gas across the cross-sectional area of the reactor. A polymerization catalyst, comprising a single catalyst or a combination of catalysts, simultaneously is introduced into the polymerization zone. Large numbers of suitable catalysts are shown in the prior art. See, for example, U.S. Pat. Nos. 4,302,566, 4,303,771, 4,383,095, and 4,427,573.
At start-up, the polymerization zone of the reactor is pre-charged with a bed of polyolefin product particles. This polymer bed is maintained in a fluidized state by the polymerizable gas/catalyst mixture rising upwardly therethrough. During operation of the reactor, new polymer product continuously is formed by the catalyzed polymerization of the olefin gas and product continuously is withdrawn to maintain the fluidized polymer product bed at a substantially constant level. The withdrawn product moves into purge hoppers and storage bins, where any unreacted alpha-olefins gas is purged by a sweeping flow of an inert gas, such as nitrogen. The inert gas and the unreacted alpha-olefins may be vented, flared to the atmosphere or recycled to the reactors as illustrated in U.S. Pat. No. 4,666,998.
Unreacted gas exiting from the top of the fluidized bed of polymer flows upwardly through a disengaging zone in the upper part of the reactor. A recycle stream containing unreacted alpha-olefins, along with any entrained solid polyolefin polymer and catalyst typically is withdrawn from the upper part of the disengaging zone and recycled to the plenum of the reactor after being run through a cyclone to remove particulate matter and then cooled and compressed. In prior art reactors, the disengaging zone normally is larger in diameter than the polymerization zone, so as to reduce the gas flow velocity and thereby facilitate the settling out of solid particulates.
It has been shown in the prior art that gas phase polymerization of alpha-olefins can be conducted in multiple stages. See, for example, U.S. Pat. Nos. 2,936,303, 4,048,412, 4,338,424, 4,390,669, 4,420,592, and 4,703,094 all of which disclose multiple stage polymerization systems. In such multi-stage processes, each individual stage typically is similar in basic design and operation to the gas phase polymerization system described above. The multiple stages typically are arranged in such a fashion as to require lengthy transfer lines or other complex means for transferring intermediate polyolefin polymers from one reactor to another, resulting in increased fabrication costs and an increased frequency of plugging in the intermediate polymer transfer lines. Frequent plugging in turn increases the system's operating cost.